Propeller



HIDETSUGU KUBOTA 2,717,044

PROPELLER Sept. 6 1955 Filed March 13, 1950 F25 E A Hmmsu e u KSSEQL *Ri. ATTORNEYS United States Patent PROPELLER Hidetsugu Kubota, Niigata-ken, Japan Application March 13, 1950, Serial No. 149,400

Claims priority, application Japan April 18, 1949 1 Claim. (Cl. 170-165) My invention relates to an axial screw propeller with multiple blades disposed at an equal distance apart both axially as well as angularly.

The object of this invention is to increase the etficiency of the propeller by making use of the theoretically indivisible relationship between the whole length of boss, over which the blades are disposed, and the pitch ration of each blade, whereby every blade may display equal propelling power corresponding with high speed rotation, so as to obtain the most eflicient hydrodynamic action of the respective blades.

According to the present invention an axial screw propeller is provided multiple blades disposed at an equal distance apart both axially as well as angularly, the foremost blade, intermediate blades and rearmost blade being disposed over the total length of the boss which is twice as long as the root length of each blade, and the respective intermediate blades having progressively increasing pitches as compared with the pitch of the foremost blade, but the pitch of the rearmost blade remaining substantially equal to that of the intermediate blade adjacent thereto.

In this specification the term induction means the mutual blade interference, that is to say, the negative pressure on the rear side of the blade due to the suction effect of the next blade.

The term slip stream means the water stream produced at the rear of the blade by reason of the difference between the mean geometrical pitch of the blade and the actual axial distance moved through by the blade during one revolution.

The term slip means loss of propelling power due to the slip stream.

Boss-length ratio means the proportion that the root length of each blade bears to the total length of the boss.

It has been known that neither the hitherto known double-blade type of propellers nor axial propellers (that is, propellers having tandem blades with successively increasing pitch) are suitable for high speed rotation. With respect to both internal combustion engines and turbines, however, the efliciency thereof increases as the speed becomes higher. Therefore all of these engines have hitherto driven under sacrifice of efficiency.

According to the present invention, aforesaid known disadvantages can be avoided and a highly efiicient propeller suitable for high speel rotation can be obtained.

The important conditions for axial propellers with multiple blades are that the pitch of each blade is properly determined in such wise that slip stream and induction are taken into account, and that also the most adequate boss-length ratio is determined having regard to the co-operating slip stream and induction of each blade.

The invention is illustrated by an embodiment in the accompanying drawings in which Fig. l is a side view of the propeller according to the invention and Fig. 2 shows efiiciency curves.

Referring now to the drawings in which the propeller 2,717,044 Patented Sept. 6, 1955 ice provided with three blades, that is to say, the foremost blade 1, intermediate blade 2 and rearmost blade 3, each having different blade surface, is illustrated, if said propeller is driven so as to rotate and propel in a fluid, the propelling power of the foremost blade will be somewhat reduced under the influence of the induction of the intermediate blade, the pitch of which is larger. The intermediate blade 2 will be influenced by both the slip stream of the foremost blade 1 and the induction of the rearmost blade 3. Therefore, in order to make this intermediate blade display a propelling power equal to that of the foremost blade, its pitch must be increased. The rearmost blade 3 will be affected by the slip stream brought about by the intermediate blade but at its back is free from the influence of induction. Therefore if the rearmostblade is to be in equal pitch to that of the intermediate blade 2, each blade must display an equal propelling power.

The present invention has been deduced from theoretical grounds based upon the effect of both induction and slip stream and is characterized by the curve of pitch of the blades progressively increasing from front towards the back of the propeller, except that the rear blade has the same pitch as the preceding one. On the other hand, the theoretical ground has heretofore been based upon the slip stream only and the pitch has been selected so as merely to increase progressively, in consequence of which if the propeller be subjected to torque of such an extent as to be suitable for the rearmost blade of high pitch, the propelling power of both the foremost blade and intermediate blade will necessarily be reduced. On the contrary, if these blades are to display their normal propelling power, the torque on only one, the rearmost blade will naturally increase, as the result of which the propelling will be done by the foreward blades and the rear blade will have no effect on the propulsion.

Thus, with the hitherto known type, it is impossible to let each blade display an equal propelling power. Ultimately unless aforesaid propeller is made of a shape near to that of the ordinary double blade type, the efficiency thereof must be so reduced that it is not suitable for high speed rotation, as stated above.

According to the present invention, however, the pitch of the propeller has been deduced from theoretical ground based upon both the slip stream and induction,

and the pitch ratio of every blade has been selected so as to form curves respectively increasing in pitch progressively from the front towards the back of the propeller except that the rearmost blade has the same pitch as the preceding one, whereby the propeller thus formed is suited for high speed rotation and the rearmost blade displays propelling power equal to that of each foremost and intermediate blade.

This can be explained as follows:

If the pitch radio of the foremost blade 1 is selected to be of 1.0, the pitch ratio of the intermediate blade 2 will become 1.0 plus slip 0.2, that is 1.2. With respect to the rearmost blade 3, the pitch ratio of this should be chosen at 1.2, because it is free from the action of induction 0.2. As far as the hydrodynamic action of each blade is concerned, the foremost blade is affected by the induction of the intermediate blade 2, which is influenced by both the slip stream of the foremost blade and the induction of the rearmost blade 3, but the rearmost blade is free from the induction behind it. Therefore it is useless to make the pitch of the rearmost blade progressively larger by 0.2 than that of the intermediate blade, and such a measure will rather present unfavourable efiects. According to the present invention, therefore, the pitch of the rearmost blade is always equal to that of the blade adjacent thereto, that is to say, the pitch ratio of every blade is selected to generate such a curve 6 of pitch as to increase progressively by degrees except that the rearmost blade has the same pitch as the preceding one, as mentioned above.

Referring now to the length of boss, this will be explained as follows:

The slip for the highest efiiciency is selected at 20%, that is, 0.2 of the root length of each blade. The induction value is considered to coincide with the said slip. If the sum of the slip and induction of each blade is equal to 02-1-02, that is, 0.4, of the root length, then the total sum for three blades amounts to 0.4x 3, that is, 1.2 of the root length of a blade. However, the rearmost blade is free from induction so that the aforementioned value is held at 1.0. This means that the arrangement of three blades in a length corresponding to twice the root length L of each blade is theoretically reasonable. Iowever, according to experiment in a water tank, when the boss length was selected at 2.1 times as long as the length L, there resulted an error of one twentieth. Further experiment has shown that when boss length was less than twice the length L, the propeller form approximated to the double-blade type, and the efiiciency was lowered, and that on the contrary when it was more than twice, the efiiciency has lowered also.

As shown on the diagrams in Fig. 2 representing the effect of the invention ascertained experimentally on the three blades, the curve A is for the propeller according to the present invention. The abscissa represents velocity whilst the ordinate represents the efficiency, and the curves show that in the case of the pitch ratio of three blades being selected at 1.0, 1.2 and 1.2 and the whole length 1 of boss being made just twice the root length L, maximum efiiciency can be obtained. The curves B and C show that when the pitch ratios of three blades are selected as mentioned above but the boss length is made 2.5 times and 1.5 times respectively, the efficiency in each case is lowered as compared with the case A. The curves a, b, and 0 show that as compared with the curve A of this invention, the efiiciency is respectively lowered too, in the case of the pitch ratios of the propeller being selected to increase by degrees such as 1.0, 1.2 and 1.4

4 respectively and of whole length of boss being made 2, 2.5 and 1.5 times the length L.

According to the present invention, the pitch ratio of each propeller blade is selected so as to form curves having pitches that respectively increase progressively from the front to the rear except that the pitch of the rearmost blade is equal to that of the preceding blade, and the length over which the blades are disposed, that is to say, the whole length of the boss, is theoretically calculated so that high speed rotation and high efliciency may be obtained.

What I claim as my invention is:

A propeller of the class described, comprising: an elongated forwardly and rearwardly extending boss member; a plurality of propeller blades disposed along said boss member longitudinally thereof at equal distances therealong and at equal angular displacements from each other, the pitch of each blade intermediate the foremost blade and the rearmost blade increasing by a constant increment from the pitch of the adjacent blade immediately forward thereof, the pitch of the rearmost blade being substantially equal to the pitch of the adjacent blade immediately forward thereof, whereby the propelling power produced by each of the blades is substantially constant, said blades having equal root lengths measured longitudinally of the boss member, and the length of the boss member being substantially equal to twice the said root length of each of the blades.

References Cited in the file of this patent UNITED STATES PATENTS 33,459 Wappich Oct. 8, 1861 68,830 Atwell Sept. 17, 1867 858,749 Oldenburg July 2, 1907 1,021,822 Broussouse Apr. 2, 1912 1,445,467 Baer Feb. 13, 1923 1,479,052 Boening Jan. 1, 1924 2,116,054 Weichwald May 3, 1933 FOREIGN PATENTS 17,629 Great Britain of 1907 134,929 Great Britain Nov. 13, 1919 

